DESC:FIELD OF THE INVENTION

The present invention relates to ultra low carbon high strength cold rolled continuously annealed steel sheet with tensile strength of atleast 450 MPa having composition comprising in terms of weight %: C: =0.003, Mn: 0.75-0.95, Si: =0.03 , P: 0.06-0.09, Nb: 0.01-0.03, V: 0.003-0.02, N: = 0.004 and balance being Fe and other unavoidable impurities and a method of producing the same. Further the steel sheet provided has excellent drawability with r-bar value =1.5, excellent phosphatability and surface property with phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface, Bake hardening index of less than 5 MPa, superior room temperature aging resistance with no YPE after accelerated aging and ductility brittleness transition temperature (DBTT) less than -50 0C, suitable for automotive application.

BACKGROUND OF THE INVENTION

The recent trend in automobile manufacturing is to incorporate higher fraction of high strength steel grades in their body and structure components. High strength steel with minimum 450 MPa of tensile strength is having relevance in terms of replacing conventional 270 MPa tensile strength level ultra low carbon steels for body parts application. Also 450 MPa ultra low carbon steel can replace conventional plain C-Mn grades used for automotive structural application. 440 MPa strength level plain carbon manganese grades such as JSC440W as per JGS A 2001:2008 has its limitation as far as drawability and weldability are concerned. Low drawability of JSC440W grade is attributed to its high C and Mn weight % and higher pearlitic phase in its microstructure. To avoid that, recent trend is to alloy conventional ultra low carbon steel with high amount of solid solution strengthening elements to achieve the desired UTS level. However such amount to solid solution strengthening may deteriorate the drawability, surface phosphatability and DBTT property.

As a part of prior art Indian patent specification number 3165/MUM/2012 teaches manufacturing of a 440MPa strength level cold rolled steel sheet having excellent deep drawability having in terms of weight %: C: =0.005 %, Mn: 1- 2.5 %, Si: 0.1-1% , P: =0.1 %, S: 0.02 % , N: = 0.01 % , Al: =0.1%, Ti: 0.005-0.05%, Nb: 0.01-0.08 % and balance being Fe and other unavoidable impurities. The steel sheet described in patent application number 3165/MUM/2012 may attain a good drawability however the surface phosphatability and DBTT value is poor due to higher Mn and Si weight %.

The present invention thus makes an attempt to solve the problems of the prior art by providing an ultra low carbon high strength cold rolled continuously annealed steel sheet having UTS= 450 MPa. Provided steel sheet has superior drawability, Low yield ratio, excellent aging resistance, and good surface and Phosphating properties with adequate resistance to low temperature brittleness through low DBTT Temperature. The transition from ductile to brittle can be very rapid and often times it could be very disastrous since there is almost no warning. A material with very low DBTT provides adequate resistance to low temperature brittleness. The present invention also relates to a process for manufacturing the said steel sheet.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide ultra low carbon high strength cold rolled steel sheet with selective composition to ensure superior drawability, low yield ratio, excellent aging resistance, and good surface and Phosphating properties with adequate resistance to low temperature brittleness. and a process for its production through continuous annealing route with controlled process parameters.

A further object of the present invention is directed to provide ultra low carbon high strength cold rolled steel sheet with selective composition produced through continuous annealing route which would ensure a tensile strength of atleast 450MPa suitable for automobile application with the advantage of weight reduction.

A still further object of the present invention is directed to provide ultra low carbon high strength cold rolled steel sheet with selective composition produced through continuous annealing route which would ensure excellent formability(r-bar >1.5) and drawability as well as improved weldability and surface property by maintaining desired lower levels of carbon, silicon and manganese in composition while still ensuring desired level of strength and no YPE after accelerated aging and DBTT less than -50 0C .

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a cold rolled continuously annealed steel sheet with tensile strength of atleast 450MPa comprising in terms of weight %:

(In weight %) (In weight %)
C: =0.003 Si: =0.03
Mn: 0.75-0.95 V: 0.003-0.02
Nb: 0.01-0.03 P: 0.06-0.09
N: 0.004 or less

and balance being Fe and other unavoidable impurities whereas,
Elemental composition of [Mn+Si] / [P+Nb+V] must be in the range of 6 to 13 and %[Nb+V] / %[C+N] must be in the range of 2.5 to 8.5, where [X] is weight % of steel element X having YS/UTS ratio =0.65, mean planer anisotropy ratio (r-bar) of =1.4, bake hardening index of less than 5MPa, and no yield point elongation after accelerated aging for atleast 12 months.

A further aspect of the present invention is directed to cold rolled continuously annealed steel sheet where as the steel sheet further contains Ti and B in an amount such that
5=%[Nb+Ti+V]/ %[N+C+B]=20,
And, %[Ti+B] = %[3N* +0.8N]
Where N* = (14/48) x [Ti] wt%, N=Total N wt%, [X] = weight % of element X.
having excellent phosphatability and surface property with phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface.

A still further aspect of the present invention is directed to said cold rolled continuously annealed steel sheet having bake hardening index of less than 5 MPa, superior room temperature aging resistance with No YPE after accelerated aging and DBTT less than -50 0C.

Advantageously, said cold rolled continuously annealed steel sheet is having ductile to brittle transition temperature (DBTT) less than -50 °C.

Another aspect of the present invention is directed to said cold rolled continuously annealed steel sheet having complete ferritic microstructure with precipitates of nitride and carbide and sulphide forming elements where as ferrite grains having an average ferrite grain sizes are in the 8 to 15 micron.

Yet another aspect of the present invention is directed to said cold rolled steel sheet comprising by mass % atleast one type of element selected from the group comprising Zr, Mg, Cr, Mo, W, Hf, Co, Ni, Cu, Zn, Sc, Ca, Pb and Sn such that each element by content in the range of 0.002% to 0.025 %.

A further aspect of the present invention is directed to a process for manufacturing cold rolled steel sheet having chemical composition comprising the processing steps of:
a.) reheating the slab having said composition to reheating temperature of 1160 °C -1220 °C;
b.) said Reheated slab being roughing rolled in roughing mill with roughing mill delivery temperature of 1060°C or less ;
c.) said rough rolled steel being subjected to finish rolling after at temperature range of 860°C to 920°C;
d.) coiling the finish rolled steel at with average run out table cooling rate of 8°C/second - 15°C/second;
e.) cold rolling the said hot rolled steel sheet with cold reduction of 55% or more.

A still further aspect of the present invention is directed to said process comprising annealing said cold rolled steel sheet following the steps of:
a) annealing at soaking section temperature range of 760 °C to 810°C with residence time of for 40 to 110 seconds;
b) slow cooling the steel up to a temperature range of 670°C to 720°C after soaking ;
c) rapid cooling the steel up to a temperature range of420 °C to 500°C with cooling rate of 10°C / second to 30°C / second;
d) overaging said steel at temperature range of 320°C to 400°C; and
e) subjecting the overaged steel to skin pass elongation of 0.6% to 1.3 %.

A still further aspect of the present invention is directed to said process comprising after hot rolling hot rolled coils subsequently processing through pickling coupled with tandem cold rolling mill to remove the oxide surface present in the surface and to provide cold reduction of 55% or more.

A still further aspect of the present invention is directed to said process wherein the said steps are controlled such as to provide selectively end characteristics in the steel sheet having:
UTS =450MPa, YS/UTS ratio =0.65, mean planer anisotropy ratio (r-bar) of =1.4, bake hardening index of less than 5MPa, and no yield point elongation after accelerated aging for at least 12 months;
Ductile to brittle transition temperature (DBTT) less than -50 °C;
phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface; and
complete ferritic microstructure with precipitates of nitride and carbide and sulphide forming elements where as ferrite grains having an average ferrite grain sizes are in the 8 to 15 micron.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to illustrative examples.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO EXAMPLES INCLUDING A PREFERRED EMBODIMENT

The present invention is directed to providing a cold rolled continuously annealed 450MPa tensile strength level steel sheet having excellent drawability with r-bar value =1.5, Excellent phosphatability and surface property with phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface, superior room temperature aging resistance combination with no YPE after accelerated aging and DBTT less than -50 0C. Steel has in terms of weight % , C: =0.003, Mn: 0.75-0.95, Si: =0.03 , P: 0.06-0.09 ,Nb: 0.01-0.03 , V: 0.003-0.02, N: = 0.004 and balance being Fe and other unavoidable impurities whereas Elemental composition relation having [Mn+Si] / [P+Nb+V] must be in the range of 6 to 13 and %{ [Nb+V] / %[C+N] } must be in the range of 2.5 to 8.5, where [X] is weight % of steel element X . Cold rolled continuously annealed steel further contains Ti and B in an amount such that 5 = {[Nb+Ti+V] / [N+C+B]} = 20, and [Ti+B] = [3N* +0.8N] Where N* = (14/48) x [Ti] wt%.

Role of elemental compositions and the detail processing method for the high strength cold rolled continuously annealed steel sheet of 450MPa tensile strength level as per the present invention will be described in detail as follows:
Following abbreviations have been used to describe the present invention:
SS – Soaking section in continuous annealing furnace
SCS – Slow cooling section section in continuous annealing furnace
RCS- Rapid cooling section in continuous annealing furnace
OAS- Over aging section in continuous annealing furnace
YS – Yield strength in MPa
UTS- Ultimate tensile strength in MPa
BH – Bake hardening index in MPa
SPM %– Skin passes elongation in %
YP- Yield Point
YPE- Yield Point Elongation %
El%-Total Elongation at 50mm gauge length sample
CCM%- Cold rolling reduction %
DBTT- Ductile-Brittle Transition Temperature

Carbon (=0.003wt%) - C present as intestinal solute is a major element deteriorating the drawability as well as increasing the strength of steel. It is well known that lower the C wt% softer will be the material and aging property and drawability will be better .Furthermore, reducing the C wt% reduces the amount of alloying addition (Nb, Ti, V) to fix the same ultimately reducing the production cost . Also C in steel as an interstitial element prevents the formation of {111} texture, deteriorating the drawability. Hence to get r-bar value =1.5 and BH =5 MPa, the C wt% is restricted to 0.003wt% or less.

Manganese (0.75 to 0.95 wt %): Manganese is an effective solid solution strengthening element which facilitates achieving the desired level of Tensile strength of 450 MPa or more. Effect on Yield strength is not significant hence Mn helps in achieving desired yield ratio of 0.65 or less. However, to achieve any desired strengthening effect the minimum level should be more than 0.7 wt%, and therefore its lower limit should preferably be atleast 0.75 wt%. Keeping the Mn wt% to higher value significantly hampers the drawing property (? value) and phosphatability of interstitial free high strength rephosphorized steel hence upper limit is restricted to 0.95 wt% for present steel.

Phosphorus (0.06 to 0.09 wt%)- Phosphorus is most important strengthening element in present invention as the name itself implies as an Interstitial free high strength rephosphorized steel. P is as an element which improves the strength at low cost, and the amount of addition thereof varies depending on a target strength level. For the strength level of more than 450 MPa as described in scope of present invention the minimum amount of P should be more than 0.06 wt%. However when added amount exceeds more than 0.1 wt% the DBTT value suffers due to higher phosphorus segregation at the grain boundary.

Silicon (=0.03 wt %) – Silicon is an element utilized for increasing the strength of steel. As the silicon content increases the ductility and r-value noticeably deteriorated. Since silicon deteriorates plating / phosphatability properties by forming SiO2 type of oxides (Scale). It is advantageous to add as low an amount of silicon in the steel as is possible, the added amount of silicon is preferably 0.03 wt% or less.

Niobium(0.01-0.03wt%): Nb acts a carbide former and fix the unbound carbon present in steel. In that sense Nb also increases the strength by forming nano metric NbC precipitate which restricts the grain growth during hot rolling and annealing. NbC precipitates also helps in developing favorable {111} oriented grains for good r-bar value. To achieve the said effects minimum Nb weight % must be atleast 0.01 %. Adding Nb more than 0.03 weight % results in addition cost of production and higher yield ratio.

Titanium and Boron- Titanium is added to fix carbon and nitrogen to make steel interstitial free. Boron is added as a nitride former .However when Ti and B are added in combination to Nb and V their amount should be optimized based on the level of total N And C present in steel and Nb, V added in combination of Ti and B as they are Carbide and Nitride formers. To achieve the maximum effect and to avoid any free Ti and B in solution the weight % of Ti and B has been controlled based on the following relation, {5 = % [Nb+Ti+V] / % [N+C+B] = 20}.

Nitrogen (0.004wt% or less) -The upper limit for N is 0.004%. It is advisable to keep it to minimum level. Higher N content requires to higher Ti, V and B addition to fix extra N and increase the volume of TiN, VN precipitates which strengthens the material, ultimately deteriorating the drawing property.

Vanadium (0.003 -0.02 wt%)- Vanadium has been added with intension to fix solute Carbon as VC. Vanadium is weak carbide former than Titanium, makes the grain size coarser. It has been reported that Ti and V added steel has increased Elongation and r bar of the steel than titanium only added steel. To achieve the said advantage scope of V level is from 0.003 -0.02 wt% in present inventive steel.

Zr, Mg, Cr, Mo, W, Hf, Co, Ni, Cu, Zn, Sc, Ca, Pb and Sn – Present inventive steel further includes atleast one from Zr, Cr, Mg, Mo, W, Hf, Co, Ni, Cu, Zn, Ca, Pb and Sn such that each element by content in the range of 0.002% to 0.025 %. These elements are included as they form carbide and/or nitride and/or sulphide which support the aging resistance and low BH index. However amount more than 0.025 add up to the cost of production and also reduces the drawability.

Complete description of the manufacturing process:

To achieve Slab chemistry as described in scope of the invention Heat from basic oxygen furnace (BOF) is processed through RH degasser and subsequently continuously casted. Special measure have been taken to hot roll resulted slabs by keeping slab reheating temperature in the range of 1160°C to 1220°C intended to control roughing mill delivery temperature under 1060°C and finishing mill entry temperature under 1020°C to check surface defects like rolled in scale. During hot rolling finishing mill temperature range of 860°C to 920°C and run out table cooling rate from finishing mill to coiler in the range of than 8-15 0C/sec was maintained. After hot rolling hot rolled coils were subsequently processed through pickling coupled with tandem cold rolling mill to remove the oxide surface present in the surface and to provide cold reduction of 55% or more.

Following pickling and cold rolling to desired thickness, cold rolled steel sheet being processed through continuous annealing line, where electrolytic cleaning removes rolling emulsion present on the surface. Cleaned surface passes through the preheating and heating section where the strip is heated at the rate of 1-10 0C/sec to soaking section temperature maintained at 760 °C -810 °C. Annealing time of 40-110 seconds gives desired results for present bake hardenable steel. After soaking steel strip passes through slow cooling section at cooling rate of less than 3°C/sec. Slow cooling section temperature of 670 °C - 720°C was maintained. Following slow cooling section annealed strip sheet been rapid cooled at 10 °C/sec or more up to rapid cooling section temperature maintained in the range of 420-500 0C. After rapid cooling section annealed strip was over aged keeping the over aging section temperature of 320°C -400 °C for 100 to 300 seconds. Subsequent to over aging steel strip is given a skin-pass elongation (temper rolling) in the range of 0.6% to 1.3% to avoid yield point elongation and to improve flatness of steel strip.

Furthermore, cold rolled high strength steel sheet with UTS=450 MPa described in present invention can be processed through continuous galvanizing route for zinc coating to produce GA/GI steel sheets and used as coated product for similar applications.

Method of evaluating bake hardening in a tensile test:

Tensile test specimen as per JIS Z2241 No.5 with 50mm gauge length 25mm width was and prepared across the rolling direction of steel sheet. Tensile test specimen was then strained to 2% at strain rate of about 0.008/second and then heated at 1700C for 20 minutes. Heated sample was then subjected to tensile test. Bake hardening index was then evaluated by measuring the difference between the initial strength at 2% strain before bake hardening and final yield strength (at lower yield point) after heating at 1700C for 20 minutes.

Method of evaluating phosphatability:

Phosphating process is a chemical the treatment of a metal surface which gives reasonably hard, electrically non-conducting surface coating of insoluble phosphate which is contiguous and highly adherent to the underlying metal and is considerably more absorptive than the metal which provides excellent corrosion resistance and paint ability to steel surface .The coating is formed as a result of a top chemical reaction, which causes the surface of the base metal to integrate itself as a part of the corrosion resistant film. [1]
[1]- T.S.N. Sankara Narayanan, SURFACE PRETREATMENT BY PHOSPHATE CONVERSION COATINGS, Rev.Adv.Mater.Sci. 9 (2005) 130-177

To evaluate phosphatability firstly alkali degreasing was performed on steel sheet at 400 C for 120 sec using FC-E2032 chemical manufactured by NIHON PARKERIZING India Pvt Ltd to the obtained cold rolled steel sheet without any oil/grease on surface. Degreasing was followed by water rinsing and then surface conditioning at room temperature for 30 seconds using PL-Z chemical manufactured by NIHON PARKERIZING India Pvt Ltd. Phosphate treatment using PB-L3020 chemical, manufactured by NIHON PARKERIZING India Pvt was done at 400 C for 90 seconds. Subsequently, the surface after phosphate treatment was observed under a Scanning electron microscope using Secondary Electron image mode. Average grain size was measured assuming circular phosphate crystals. Crystal size < 4µm is considered as excellent for phosphatability. The phosphate coating weight was measured using the XRF method and steel sheet with average coating weight after zinc phosphate chemical conversion coating of 1.5-2.5 g/m2 is considered having excellent phosphatability.

Complete description of Inventive steel and comparative steel grades are illustrated from table 1 to 3 as follows:

Table-1: Chemical compositions of example and comparative steel sheets with values of elemental relationships of constitutional elements.

Table-2: Hot rolling, cold rolling and annealing parameters of inventive and comparative steel sheets having chemical compositions as per table 1.

Table-3: Mechanical properties, surface phosphatability property, DBTT temperatures, bake hardening index and artificial aging property of inventive and comparative steels having chemical composition as per table 1 and being processed as per table 2.

As can be witnesses from Table 1 and 2, Casting and processing of Steels remarked as “ I ” are carried out by strictly controlling the content of C, Mn ,P, B, Ti, Si, N, V and Nb to satisfy the condition of C: =0.003, Mn: 0.75-0.95, Si: =0.03 , P: 0.06-0.09 ,Nb: 0.01-0.03 , V: 0.003-0.02, N: = 0.004 and balance being Fe and other unavoidable impurities whereas elemental composition relation having {[Mn+Si] / [P+Nb+V]} in the range of 6 to 13 and {[Nb+V] / [C+N]} in the range of 2.5 to 8.5 is satisfied . Inventive steel sheet shave Ti and B in an amount such that 5 = {[Nb+Ti+V] / [N+C+B]} = 20, and [Ti+B] = [3N* +0.8N] Where N* = (14/48) x [Ti] wt%. Further, As listed in table 3 steels remarked as “I” have excellent drawability with r-bar value =1.5, excellent phosphatability and surface property with phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface, Bake hardening index of less than 5 MPa, superior room temperature aging resistance with No YPE after accelerated aging and DBTT less than -50 0C.

TABLE -1
Steel No C,
wt% Mn,
wt% P,
wt% Si,
wt% N,
wt% Nb,
wt% V,
wt% Ti,
wt% B,
wt% [Mn+Si] / [P+Nb+V] [Nb+V] / [C+N] [Nb+Ti+V] / [N+C+B] [Ti+B] [3N* +0.8N] Remarks
1 0.001 0.95 0.07 0.008 0.003 0.02 0.005 0.04 0.0012 10.1 6.3 12.5 0.041 0.037 I
2 0.0012 0.8 0.07 0.008 0.0025 0.015 0.004 0.03 0.0014 9.1 5.1 9.6 0.031 0.028 I
3 0.0015 0.96 0.06 0.01 0.003 0.02 0.01 0.045 0.0019 10.8 6.7 11.7 0.047 0.042 I
4 0.003 0.7 0.08 0.02 0.0035 0.025 0.008 0.035 0.001 6.4 5.1 9.1 0.036 0.033 I
5 0.02 1.4 0.042 0.9 0.0026 0.018 0.004 0.018 35.9 1.0 1.8 0.018 0.018 C
6 0.003 0.3 0.04 0.01 0.004 0.02 0.01 0.03 4.4 4.3 8.6 0.030 0.029 C
7 0.003 0.7 0.17 0.2 0.003 0.02 0.01 0.03 0.001 4.5 5.0 8.6 0.031 0.029 C
8 0.002 0.3 0.04 1.5 0.004 0.03 0.005 0.015 0.0015 24.0 5.8 6.7 0.017 0.016 C
9 0.003 0.75 0.06 0.008 0.007 0.002 0.003 0.003 11.7 0.5 0.8 0.003 0.008 C
10 0.002 2.2 0.07 0.2 0.002 0.02 0.035 0.0014 26.7 5.0 10.2 0.036 0.032 C

* I- Present inventive example, C- Comparative Examples
* All compositional elements are in terms of weight %.
* Underlined and shaded boxes indicates “outside the appropriate range”.
* Steel No’s having value of [Mn+Si] / [P+Nb+V] outside the range of 6 to 13 do not comply with either strength or phosphatability and DBTT requirement. Mechanical properties, Phosphatability and DBTT results for the same have been listed in table number 3.
* Steel No’s having [Nb+Ti+V] / [N+C+B] ratio in the range of 5 to 20 comply with the room temperature aging requirement. YPE after artificial aging at 100 0C for 6 hours for inventive and comparative steel sheets are listed in table 3.

TABLE -2
Steel No FT,
0C ROT cooling rate ,
0C/sec CT,
0C SS,
0C SCS,
0C RCS,
0C RCS
Cooling
Rate,
0C/sec OAS,
0C SPM,
% AT,
Sec CCM,
% REMARKS
1 A 910 11.5 645 780 700 481 18.4 370 0.9 49 68 I
1 B 915 11.9 632 850 721 493 6.9 370 0.9 150 68 C
2 A 920 10.2 648 794 690 459 17.6 381 1 52 65 I
2 B 869 6.2 720 800 710 459 15.7 370 1.2 71 65 C
3 915 12.6 640 770 675 490 16.2 379 1.1 54 70 I
4 920 10.5 621 800 697 478 16.3 364 0.85 60 68 I
5 880 12.4 620 780 720 480 16.9 387 1.2 68 62 C
6 900 8.9 630 792 684 468 12.5 398 0.8 83 75 C
7 870 9.3 625 785 695 457 17.9 369 1.2 54 65 C
8 897 8.7 653 801 705 476 15.8 382 1 68 65 C
9 905 12.3 633 789 685 463 15.4 376 0.8 73 63 C
10 860 11.4 620 812 691 475 16.5 361 1.2 51 68 C

* I - Present inventive example, C - Comparative Examples
** Underlined and shaded boxes indicates “outside the appropriate range”
* Steels No’s marked as 1A and 1B have the same chemical composition as steel No.1 but processed at different hot rolling and annealing conditions. Similarly steel number 2A and 2B have the same chemical composition as steel No 2.
* FT- hot finish rolling temperature , ROT- run out table in hot strip mill , SS- soaking section ,SCS- Slow cooling section , RCS- Rapid cooling section , OAS- Overaging section , SPM- Skin pass elongation ,CR – Cold rolling reduction % ,AT- Annealing Time

TABLE -3
Steel No YS, MPa UTS, MPa YS/UTS Total El % n Value r-bar BH
Index,
MPa Ferrite Grain Size,
µm DBTT,
0C Phosphatability
Remark YPE %,
(After 9 hours of aging at 100 0C) Remarks
1A 290 468 0.62 38.1 0.2 1.6 0 9.4 -60 o 0 I
1B 257 391 0.657 39.3 0.2 1.65 0 12.2 -60 ? 0 C
2A 284 452 0.63 38.7 0.203 1.7 0 8.9 -60 o 0 I
2B 259 401 0.64 38.5 0.2 1.65 0 12.5 -50 ? 0 C
3 283 466 0.61 37.8 0.2 1.6 0 10.2 -60 o 0 I
4 279 454 0.61 38.6 0.203 1.8 0 10.5 -60 o 0 I
5 380 520 0.73 24.4 0.15 1.02 23 6.5 -30 ? 0.9 C
6 183 324 0.56 44.9 0.22 1.75 0 22.3 -70 o 0 C
7 298 484 0.62 36.9 0.19 1.25 0 8.5 -10 ? 0 C
8 323 483 0.67 34.7 0.18 1.2 5 8.9 -40 ? 0 C
9 279 442 0.63 37.3 0.2 1.5 51 10.2 -60 o 1.7 C
10 302 491 0.62 31.3 0.16 1.3 0 8.1 -20 ? 0 C
* I - Present inventive example, C - Comparative Examples
* O - Steel shows good phosphatability after zinc phosphate chemical conversion coating having phosphate crystal size less than 4µm and phosphate coating weight in the range of 1.5-2.5 g/m2 ,
? - Steel shows poor phosphatability after zinc phosphate chemical conversion coating having phosphate crystal size more than 4µm and phosphate coating weight more than 2.5 g/m2 ,
* Steel sheets having DBTT more than -50 0C does not comply with the scope of the invention susceptible to low temperature embrittlement.
* To simulate the room temperature aging tensile test specimen of 50mm gauge length was immersed in oil bath homogeneously maintained at 1000C for 9 hours to simulate atleast 12 months aging .Aged sample were subjected to tensile test. Steels showing No YPE after 12 months of artificial aging and subsequent tensile test fulfills the scope of the invention and comply with twelve months of aging guarantee.
* “I” Steel sheets have the average ferrite grain size in the range of 8 to 15 micron.
The present invention and its scope is further described with the help of following illustrative examples:

Example 1: It can be appreciated from Table-1 to Table-3 that steel No. 1A,2A,3,4 marked as “I” are satisfying all the scope of the present invention and shows good drawability (r-bar=1.5), Excellent phosphatability with phosphate crystal size <4µm post zinc phosphate chemical conversion coating, good resistance to temper embrittlement with DBTT value less than -50 0C ,improved aging resistance to artificial aging with no YPE post artificial aging in oil bath for 9 hours maintained at 100 0C , ferrite grain size in the range of 8 to 15 micron , minimum Tensile strength of 450 MPa with yield ratio of =0.65 and BH index of less than 5 MPa .

Example 2: Steel sheet number 1A and 1B (listed in Table-2) have the same chemical composition as steel No 1 (listed in Table -1).However steel No 1B has been processed keeping the higher annealing time of 150 seconds and lower cooling rate of 6.9 0C/sec (Table-2). As a result steel sheet number 1B shows lower UTS value of 391 MPa against the minimum specified value of 450MPa (Table-3).Also the phosphatability of steel sheet 1B is rather poor due to very high annealing time.

Similar observation can be made for steel number 2B having same chemical composition as steel No 2A but being processed at different Hot rolling condition than steel 2A. Steel 2B is processed at very high coiling temperature of 720 0C with very low ROT cooling rate of 6.2 0C/sec. consequently, due to excessive surface oxides formation; steel 2B shows rather poor phosphatability. Also, owing to high Coiling temperature, steel 2B has lower UTS of 401 MPa than specified target value of 450MPa.

Example 3: Steel number 5 shows poor phosphatability as a result of high Mn (=1.4) and Si (=0.9) weight % (Table-1). Also steel sheet number 5 doesn’t comply either with [Mn+Si] / [P+Nb+V] in the range of 6 to 13 or the relation 5 = [Nb+Ti+V] / [N+C+B] = 20. As a result steel sheet number 5 has poor DBTT value and deteriorated room temperature aging property respectively. In addition, Due to high C, Mn and Si weight % steel sheet number 5 has ferrite grain size less than the specified range of 8 to 15 micron resulting in high UTS(=520MPa) and poor r-bar value (=1.02).

Similar observation can be made for steel number 10 having 2.2 weight % of Mn and 0.2% of Si showing poor phosphatability. Also steel number 10 doesn’t satisfy the value of [Mn+Si] / [P+Nb+V] in the range of 6 to 13.

Example 4: Steel sheet number 6 shows lower UTS of 324 MPa as it has lower Mn and P content than the specified range. Also the specified range of [Mn+Si] / [P+Nb+V] is not satisfied. In addition, Ferrite grain size is too large (=22.3 micron) to attain the desired strength level of 450 MPa

Example 5: Steel number 7 shows very poor DBTT value of -10 0C due to excessive P weight % of 0.17. P segregation at ferrite grain boundary resulting in poor ferrite grain boundary strength and consequently DBTT value is poor. Also phosphatability of steel number 7 is rather poor attributable to high P and Si weight %.

Example 6: In consequence of excessive surface oxidation during hot rolling and annealing, steel number 8 with Si content of 1.5 weight % resulted in poor Phosphating property. In addition, excessive solid solution strengthening resulted in poor drawability with r-bar value of 1.2.

Example 7: Steel number 9 has a value of {[Nb+Ti+V] / [N+C+B]} outside the minimum target value of 5 (Table 1) as specified in the claim of the invention. As a result there are unbound N and C present in the steel which accounts for the poor ageing resistance and high BH index of steel no. 9. (Table 3)

It is thus possible by way of the present invention to provide ultra low carbon high strength cold rolled continuously annealed steel sheet with tensile strength of atleast 450 MPa, having excellent drawability with r-bar value =1.5, Excellent phosphatability and surface property with phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface, superior room temperature aging resistance combination with no YPE after accelerated aging and DBTT less than -50 0C, suitable for automotive application.

,CLAIMS:We Claim:

1. A cold rolled continuously annealed steel sheetwith tensile strength of atleast450MPacomprising in terms of weight %:

(In weight %) (In weight %)
C: =0.003 Si: =0.03
Mn: 0.75-0.95 V: 0.003-0.02
Nb: 0.01-0.03 P: 0.06-0.09
N: 0.004 or less

and balance being Fe and other unavoidable impurities whereas,
Elemental composition of [Mn+Si] / [P+Nb+V] must be in the range of 6 to 13 and %[Nb+V] / %[C+N] must be in the range of 2.5 to 8.5, where [X] is weight % of steel element X having YS/UTS ratio =0.65, mean planer anisotropy ratio (r-bar) of =1.4, bake hardening index of less than 5MPa, and no yield point elongation after accelerated aging for atleast 12 months.

2. Cold rolled continuously annealed steel sheet as per claim 1 where as the steel sheet further contains Ti and B in an amount such that
5=% [Nb+Ti+V]/ % [N+C+B]=20,
And, %[Ti+B] = %[3N* +0.8N]
Where N* = (14/48) x [Ti] wt%, N=Total N wt%, [X] = weight % of element X.
having excellent phosphatability and surface property with phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface.

3. Cold rolled continuously annealed steel sheet as per anyone of claims 1 or 2 having bake hardening index of less than 5 MPa, superior room temperature aging resistance with No YPE after accelerated aging and DBTT less than -50 0C.

4. Cold rolled continuously annealed steel sheet as per anyone of claims 1 to 3 having ductile to brittle transition temperature (DBTT) less than -50 °C.

5. Cold rolled continuously annealed steel sheet as per anyone of claims 1 to 4 having complete ferritic microstructure with precipitates of nitride and carbide and sulphide forming elements where as ferrite grains having an average ferrite grain sizes are in the 8 to 15 micron.

6. Cold rolled steel sheet according to claim 1 to 5, comprising by mass % atleast one type of element selected from the group comprising Zr, Mg, Cr, Mo, W, Hf, Co, Ni, Cu, Zn, Sc, Ca, Pb and Sn such that each element by content in the range of 0.002% to 0.025 %.

7. Process for manufacturing cold rolled steel sheet having chemical composition as per claim 1 to 6 comprising the processing steps of:
a) reheating the slab having said composition to reheating temperature of 1160 °C -1220 °C;
b) said Reheated slab being roughing rolled in roughing mill with roughing mill delivery temperature of 1060°C or less ;
c) said rough rolled steel being subjected to finish rolling after at temperature range of 860°C to 920°C;
d) coiling the finish rolled steel at with average run out table cooling rate of 8°C/second - 15°C/second;
e) cold rolling the said hot rolled steel sheet with cold reduction of 55% or more.

8. Process as claimed in claim 7 comprising annealing said cold rolled steel sheet following the steps of:

a) annealing at soaking section temperature range of 760 °C to 810°C with residence time of for 40 to 110 seconds;
b) slow cooling the steel up to a temperature range of 670°C to 720°C after soaking ;
c) rapid cooling the steel up to a temperature range of420 °C to 500°C with cooling rate of 10°C / second to 30°C / second;
d) overaging said steel at temperature range of 320°C to 400°C; and
e) subjecting the overaged steel to skin pass elongation of 0.6% to 1.3 %.

9. Process as claimed in anyone of claims 7 or 8 comprising after hot rolling hot rolled coils subsequently processing through pickling coupled with tandem cold rolling mill to remove the oxide surface present in the surface and to provide cold reduction of 55% or more.

10. Process as claimed in anyone of claims 7 to 9 wherein the said steps are controlled such as to provide selectively end characteristics in the steel sheet having:
UTS =450MPa, YS/UTS ratio =0.65, mean planer anisotropy ratio (?) of =1.4, bake hardening index of less than 5MPa, and no yield point elongation after accelerated aging for atleast 12 months;

ductile to brittle transition temperature (DBTT) less than -50 °C;
phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface; and

complete ferritic microstructure with precipitates of nitride and carbide and sulphide forming elements where as ferrite grains having an average ferrite grain sizes are in the 8 to 15 micron.

Dated this the 4th day of October, 2016
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)